This invention relates to a probe for sensing the level of a material, particularly a liquid.
Although the invention is useful in sensing the levels of materials, such as liquids, in any type of application, the invention will be described in terms of sensing the filling of flammable fluids into tank trucks for purposes of explanation. When tank trucks used for hauling such fluids are filled, it is advantageous from a safety standpoint as well as from a control standpoint to sense the level of the liquid. Overfilling of such a vehicle involves not only waste but a danger of explosion resulting from sparks, static electricity or other forms of ignition. It would, of course, be possible for the operator of the filling pumps to make a visual survey of the liquiid level of the tank. However, flammable liquid handling vehicles are often compartmentalized making visual observation of a plurality of tanks impossible or at least difficult. Also, there may not be the necessary personnel in attendance to supervise the filling operation. In many applications, therefore, it is necessary to automatically sense the level of fluid within a container to terminate the filling operation or to give an indication that the tank is full.
There are many types of optical probes known in the prior art for sensing the level of materials such as liquids. Such prior art probes usually suffer two deficiencies. First, a residue is left on the proble after the liquiid has been drained away. The residue causes the probe to see fluid which is not there. Second, the cost of assembly and manufacture of the present prior art optic probes can be substantial. One type of present probe employs fiber optic bundles to channel light to and from a prism. As long as the liquid being sensed is below the prism surface, light is reflected from one fiber optic element to a second fiber optic element. When liquid impinges upon the reflecting surfaces of the prism, the index of refraction at that surface changes and the light is not reflected to the second fiber optic element but is rather transmitted through the prism and into the liquid. The prism and fiber optic terminations are epoxied into a machined aluminum carrier. The aluminum carrier is supported by a phenolic or metal tube and the exposed fiber optic cables are encapsulated in a fuel resistant or liquid resistant epoxy. The complicated assembly processes used in making these optical probes result in an expensive prism sensor.